The tricky thing about the wheel is not conceiving of a cylinder rolling on its edge. It's figuring out how to connect a stable, stationary platform to that cylinder. Wheels are the archetype of a primitive, caveman-level technology. But in fact, they're so ingenious that it took until 3500 B.C. for someone to invent them. By that time — it was the Bronze Age — humans were already casting metal alloys, constructing canals and sailboats, and even designing complex musical instruments such as harps.